B cell immune responses are regulated by a family of T cell and macrophage derived glycoproteins. Here we extend our understanding of this regulation by demonstrating the following: (1) The proliferative response of unstimulated resting B cells to T1-2 antigens shows an absolute requirement for BSF-1, can be augmented by IL-1, and suppressed by IFN-gamma. In contrast T1-1 antigens induce antigen-specific proliferation in the absence of exogenous B cell stimulatory factors. (2) Continuously growing bone marrow cell lines which are predominantly pre-B cell in nature proliferate in response to T cell and macrophage derived factors. This proliferation can be suppressed by IFN-gamma. (3) Continuously growing Lyl-positive B cell lines and clones secrete interleukin-l. (4) Lymphokines which enhance the proliferation of BCL1 tumor cells show biochemical and functional heterogeneity. (5) It is possible to produce cloned B cell hybrids which respond to BSF-1 by increased Ia expression, and which may therefore prove useful tools for analysis of BSF-1 receptor.